Much attention has been paid to a technology of forming a thin film transistor (TFT), by using a thin semiconductor film that is formed over a substrate having an insulating surface. A thin film transistor is used for a display device typified by a liquid crystal television. A silicon-based semiconductor material is known as a material for a thin semiconductor film applicable to a thin film transistor. Other than a silicon-based semiconductor material, an oxide semiconductor has attracted attention.
As a material for the oxide semiconductor, zinc oxide and a material containing zinc oxide as its component are known. Further, a thin film transistor formed using an amorphous oxide (oxide semiconductor) having an electron carrier density of less than 1018/cm3 is disclosed (Patent Documents 1 to 3).